1. Field of the Invention
The present invention relates to wireless communication circuits, wireless communication terminals and methods, recording media, and programs. Particularly, the present invention relates to a wireless communication circuit, a wireless communication terminal and method, a recording medium, and a program that serve to achieve a more favorable reception sensitivity even when a plurality of wireless communication functions is being used simultaneously.
2. Description of the Related Art
Recently, systems for managing commutation-ticket information or electronic-money information using contactless IC cards are becoming more widely used. For example, a user is allowed to pass a ticket gate only by placing a contactless IC card storing commutation-ticket information thereon in proximity to a ticket checking machine, or to pay a price of a product in terms of electronic money only by placing a contactless IC card storing electronic-money information thereon in proximity to a reader/writer.
A cellular phone is a device that is constantly carried with by a user thereof. If the cellular phone has the functionality of a contactless IC card or a contactless-IC-card reader/writer as described above, for example, as disclosed in Japanese Unexamined Patent Application Publication No. 11-213111, the user is allowed to pass a ticket gate or to pay a price of a product using the cellular phone as well as carrying out communications such as telephone calls and electronic mails, which is very convenient.
The problem, however, is that if a cellular phone includes a contactless-IC-card reader/writer that carries out communications by electromagnetic induction, an electromagnetic-interference wave generated by the contactless-IC-card reader/writer could degrade the reception sensitivity of the cellular phone (the reception sensitivity to radio waves used for calls or communications by cellular-phone functions).
For example, in the case of communications based on Personal Digital Cellular (PDC) Telecommunication System (ARIB STANDARD RCR STD-27), where communications are carried out in a 800-MHz frequency band using base-station transmission carrier frequencies (cellular-phone reception carrier frequencies) in a range of 810 MHz to 828 MHz, base-station reception carrier frequencies (cellular-phone transmission carrier frequencies) in a range of 940 MHz to 958 MHz, and a transmission/reception carrier-frequency gap of 130 MHz, the reception carrier frequency of a wireless channel of a cellular phone can be expressed by equation (1) below:(Reception carrier frequency of cellular phone)=810+0.025×n(MHz)  (1)where n is a positive integer.
On the other hand, the frequency of a transmission carrier for transmitting data thereon from a contactless-IC-card reader/writer provided in a cellular phone or the like is generally 13.56 MHz, and an electromagnetic wave having a frequency expressed by equation (2) below is generated as a harmonic component thereof:(Transmission harmonic-frequency of contactless-IC-card reader/writer)=13.56×m(MHz)  (2)where m is a positive integer.
Thus, for example, if m is 60 or 61 in equation (2), the contactless-IC-card reader/writer radiates a transmission harmonic wave having a frequency of 813.6 MHz or 827.16 MHz, respectively, as an electromagnetic-interference wave.
The electromagnetic-interference wave falls within the cellular-phone reception-carrier-frequency band (810 MHz to 828 MHz). Thus, when a wireless channel having a reception carrier frequency in the vicinity of the frequency of the electromagnetic-interference wave is being received by the cellular phone, the electromagnetic-interference wave interferes with communication in the wireless channel, causing degradation in the reception sensitivity.
That is, according to the PDC standard, substantially no selectivity is provided against an interference wave that is detuned by ±25 kHz from the cellular-phone reception carrier frequency, so that a degradation in the reception sensitivity arises in a wireless channel for which an electromagnetic-interference wave exists within ±25 kHz of the cellular-phone carrier frequency.
More specifically, when n=143, 144, 145, 686, and 689 in equation (1), the cellular-phone carrier frequency is 813.575 MHz, 813.6 MHz, 813.625 MHz, 827.15 MHz, and 827.175 MHz, respectively. These frequencies fall within the ranges of (813.6 MHz and 827.16 MHz)+25 kHz, where 813.6 MHz and 827.16 MHz are the frequencies of electromagnetic-interference waves associated with the harmonic orders of 60 and 61. Thus, the reception sensitivity is degraded when wireless channels having these frequencies as reception carrier frequencies thereof are being received.
According to IMT-2000 DS-CDMA System (ARIB STD-T63 Ver. 3.0.0), communications are usually carried out in full duplex with a wireless-signal bandwidth of 3.84 MHz, using base-station transmission carrier frequencies (cellular-phone reception carrier frequencies) in a range of 2,110 MHz to 2,170 MHz, base-station reception carrier frequencies (cellular-phone transmission carrier frequencies) in a range of 1,920 MHz to 1,980 MHz, and a transmission/reception carrier-frequency gap of 190 MHz.
Thus, a cellular-phone reception carrier frequency and a cellular-phone transmission carrier frequency has a relationship expressed in equation (3) below:(Cellular-phone reception carrier frequency)=(Cellular-phone transmission carrier frequency)+190 (MHz)  (3)
When a cellular-phone transmission wave is input to the contactless-IC-card reader/writer, the cellular-phone transmission wave is mixed with a contactless-IC-card-reader writer transmission wave due to non-linearity of devices constituting the contactless-IC-card-reader/writer circuit, generating an electromagnetic-interference wave having a frequency expressed by equation (4) below:(Frequency of EMI wave of contactless-IC-card reader/writer)=(Cellular-phone transmission carrier frequency)±(Contactless-IC-card-reader/writer transmission carrier frequency)×p(MHz)  (4)where p is a positive integer.
As described earlier, the transmission carrier frequency of the contactless-IC-card reader/writer is generally 13.56 MHz. Assigning the value and, for example, p=14 into equation (4), the frequency of the contactless-IC-card-reader writer EMI wave can be expressed by equation (5) below:(Frequency of contactless-IC-card-reader/writer EMI wave)=(Cellular-phone transmission carrier frequency)+13.56×14=(Cellular-phone transmission carrier frequency)+189.84 (MHz)  (5)
As described earlier in relation to equation (3), in communications based on IMT-2000 DS-CDMA System, a gap of 190 MHz exists between a reception carrier frequency and a transmission carrier frequency of a cellular phone. As is apparent from equation (5), the reception carrier frequency of the cellular phone falls in the vicinity of the frequency of the EMI wave generated by the contactless-IC-card reader writer. That is, EMI waves exist within the 3.84-MHz wireless-signal bandwidth.
Therefore, when communication by the contactless-IC-card-reader/writer function and communication by the cellular-phone function are being carried out simultaneously, the reception sensitivity is degraded is all the channels that can be received by the cellular phone.
As described above, in a terminal having both a cellular-phone function (PDC or IMT-2000 DS-CDMA System) and a contactless-IC-card reader/writer function, the reception sensitivity to a wireless channel being received by the cellular-phone function could be degraded depending on the operation status of the contactless-IC-card-reader/writer function.
Considering the situation that various wireless communication systems are being standardized and communication functions based on various standards are being mounted on a single terminal such as a cellular phone, the problem could become even more problematic.